1. Field of the Invention
This invention relates generally to optical transmission systems and, more particularly, to polarization control in optical transmission systems.
2. Description of the Related Art
Optical circuits, for example photonic integrated circuits (PICs), are often designed to operate on one or more specific polarization states of an optical signal. For example, PICs may be designed to perform a first function on a first polarization state of the optical signal and a second function on a second polarization state of the optical signal. These polarization states may include orthogonal modes, such as the transverse electric (TE) mode and the transverse magnetic (TM) mode. The TE and TM modes may be processed by two separate optical circuits, as part of a PIC. For example, a first circuit may be designed to operate on the TE mode of an optical signal, while a second optical circuit may be designed to operate on the TM mode of the optical signal. While the TE and TM modes are orthogonal with respect to themselves, under certain circumstances they may rotate, with respect to a longitudinal axis of a waveguide for example. For example, polarization rotation may occur in waveguide bends or curved portions. Optical circuits may include hundreds of such bends or curved portions to interconnect optical signals with the various optical elements of the optical circuit. Therefore, optical signals propagating through optical circuits having numerous curved portions may be subjected to polarization rotation due to such curved portions. Polarization rotation may lead to performance degradation in the PIC, and ultimately the associated optical system. Therefore, it may be desirable to maintain the polarization orientation of an optical signal as the optical signal propagates through an optical circuit, such as a PIC.
What is needed is an optical circuit which can maintain the polarization orientation of an optical signal as the optical signal propagates through the optical circuit, which may include one or more bends or curved portions.